Infectious Disease · Gastrointestinal Infections
The facts most likely to be tested
Amebiasis is caused by the protozoan Entamoeba histolytica, which is transmitted via the fecal-oral route through ingestion of cysts in contaminated food or water.
The classic clinical presentation of intestinal amebiasis is bloody diarrhea (dysentery) accompanied by abdominal pain and tenesmus.
Stool microscopy for trophozoites containing ingested red blood cells is the hallmark diagnostic finding for active intestinal infection.
The most common extraintestinal manifestation is an amebic liver abscess, which typically presents as a right upper quadrant pain and a solitary lesion in the right lobe of the liver.
The classic description of the liver abscess aspirate is anchovy paste consistency, which is sterile and lacks neutrophils.
Diagnosis of extraintestinal amebiasis is best confirmed via serology (EIA) or PCR, as stool studies are frequently negative in these cases.
The first-line treatment for both intestinal and extraintestinal amebiasis is metronidazole or tinidazole, followed by a luminal agent like paromomycin to eliminate remaining cysts.
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A 34-year-old male presents to the clinic with a 2-week history of intermittent bloody diarrhea and lower abdominal cramping. He recently returned from a 3-week backpacking trip in rural Southeast Asia. Physical examination reveals mild right upper quadrant tenderness without rebound or guarding. Laboratory studies show a mild leukocytosis and a stool sample reveals trophozoites with ingested erythrocytes. An abdominal ultrasound demonstrates a solitary, hypoechoic mass in the right lobe of the liver.
What is the most appropriate pharmacologic management for this patient?
Metronidazole followed by paromomycin
The patient has invasive amebiasis with a liver abscess; metronidazole is required to treat the tissue-invasive trophozoites, and a luminal agent like paromomycin is mandatory to eradicate the cyst carriage in the gut.
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Etiology / Epidemiology
Caused by Entamoeba histolytica via fecal-oral ingestion of cysts. High risk in travelers to endemic areas and sanitation-poor regions.
Clinical Manifestations
Presents as flask-shaped ulcers in the colon. Bloody diarrhea and RUQ pain (liver abscess) are classic.
Diagnosis
Stool antigen testing or PCR are preferred. Ova and parasites (O&P) exam requires 3 samples.
Treatment
Metronidazole followed by a luminal agent like Paromomycin. Avoid alcohol with metronidazole.
Prognosis
Most recover fully. Liver abscess rupture is the most feared complication requiring surgical consultation.
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Epidemiology & Etiology
Infection occurs via ingestion of mature cysts in contaminated food or water. It is highly prevalent in tropical regions with poor sanitation. Humans are the primary reservoir for Entamoeba histolytica.
Pertinent Anatomy
The parasite primarily colonizes the cecum and ascending colon. Hepatic involvement occurs via the portal venous system, leading to abscess formation in the right lobe of the liver.
Pathophysiology
Ingested cysts transform into trophozoites in the small intestine. Trophozoites invade the colonic mucosa, causing flask-shaped ulcers. They may penetrate the portal circulation to seed the liver, forming a anchovy paste abscess.
Clinical Manifestations
Patients present with bloody diarrhea, abdominal pain, and tenesmus. RUQ pain and fever suggest a liver abscess. Peritonitis indicates bowel perforation, a surgical emergency.
Diagnosis
Stool antigen testing or PCR are the most sensitive diagnostic tools. Ova and parasites (O&P) microscopy may show trophozoites with ingested RBCs. Liver ultrasound or CT is required to evaluate for abscess.
Treatment
Treat invasive disease with Metronidazole or Tinidazole. Follow with a luminal agent like Paromomycin or Iodoquinol to eliminate remaining cysts. Disulfiram-like reaction occurs if alcohol is consumed with metronidazole.
Prognosis
Prompt treatment prevents liver abscess rupture into the peritoneum or pericardium. Monitor for resolution of symptoms; persistent fever suggests treatment failure or secondary bacterial infection.
Differential Diagnosis
Bacillary dysentery: usually presents with higher fever and more acute onset
Ulcerative colitis: lacks travel history and positive stool antigen
Giardiasis: typically causes greasy, foul-smelling stools without blood
Appendicitis: localized RLQ pain without bloody diarrhea
Pyogenic liver abscess: usually polymicrobial and associated with biliary disease